I cut all these pieces to approx. 2cm x 2cm, and they mostly fired differently. The thicker (3mm) glass stayed in shape more than the 2mm glass. If you fire two 3mm pieces on top of each other, they will fire to a better shape. However, these testers are good for remembering how your different colours fire ........ but they could also be used!
Apparently all glass 'wants' to fire to 6mm - this is a rule - so some thinner glass will pull in - hence the yellow glass peanut shape - which I rather like :o).